Real-time PCR quantification of Rhizoctonia solani AG-3 from soil samples

Rhizoctonia solani anastomosis group 3 (AG-3) causes several diseases of potato, including black scurf and stem canker, affecting potato production in the Skagit Valley, Washington, and around the world. Primers for a SYBR-Green II-based real-time polymerase chain reaction (qPCR) assay were designed from sequences of the nuclear internal transcribed spacer (ITS) regions of fungal isolates of potato and onion from the Pacific Northwest, USA. The primers preferentially amplified R. solani AG-3 DNA, compared to DNA from R. solani AG-4, AG-5 and AG-8. In silico analysis of primer-template duplex stability indicated that the assay also will detect R. solani AG-3 isolates from pea and onion in Washington State and from diverse crop species around the world, but not R. solani AG-9 and AG-2-1. The assay was used to quantify R. solani AG-3 populations in pathogen-infested field soils after temporary flooding rotation, a practice found to be effective for reducing Sclerotinia sclerotiorum and R. solani AG-3 in potatoes in growth chamber studies. Population densities of the pathogen were not significantly reduced in saturated (flooded) soils relative to fallow. However, the qPCR approach was more sensitive and quantitative than the toothpick baiting method for diagnosis of these soil samples. Accurate detection and quantification of R. solani AG-3 in soil will facilitate the development of integrated management plans for Rhizoctonia diseases of potato.